Ceramic coatings can be prepared by thermal or plasma spraying and physical vapour deposition (PVD) techniques, among others. In plasma spray, a bulk powder is passed through a plasma and directed towards a substrate where it cools on contact. Films up to about 10 mm thick can be produced in this way but usually need post-deposition treatment by organic or ceramic sealants, as the deposited film tends to be porous. For example, French Patent No. 2,683,813, published Jun. 21, 1993, discloses a process for reducing porosity of plasma deposited alumina ceramic materials by using orthophosphates. The ceramic materials are formed of alumina or oxides of magnesium, or titanium dioxide. The process relates to plasma deposited ceramic technology and not sol-gel technology.
E. Kumpulainen, et al., in a paper entitled "Characteristics of Phosphoric Acid Sealed Ceramic Oxide Coatings", published 1996, discloses that phosphoric acid can be used as a sealing material for thermally sprayed alumina, chromia or zirconia coatings. K. Niemi, et al., in a paper entitled "Thermally Sprayed Alumina Coatings with Strongly Improved Wear and Corrosion Resistance", published Jun. 20, 1994, discloses that aluminum phosphate can be used in sealing the surface of thermally sprayed alumina coatings. Again, this paper relates to thermally sprayed ceramic coatings, and does not employ sol-gel technology.
Porosities of higher than 10% are not uncommon in thermal sprayed coatings. Exception is Thermal Barrier Coating, where 10%-15% porosity is beneficial in decreasing thermal conductivity of the coating. Another drawback of the plasma spray technique is that only line of sight geometries can be successfully coated. In PVD techniques, expensive vacuum systems are required to coat high quality ceramic films of less than 10 microns in thickness. This technique is also limited to line of sight geometries.
In recent years, an alternative method, called sol-gel processing, has become promising for deposition of ceramic coatings. A sol is a dispersion of solid particles in a liquid phase where the particles are small enough to remain suspended indefinitely by Brownian motion. For aqueous sols, this means a particle size less than approximately 0.1-1 .mu.m. A gel is a solid containing a liquid component in an internal network structure so that both the liquid and solid are in highly dispersed state. In the highly viscous gel state, material can be shaped into a useful product, e.g. bulk shape, fibre, coating, etc. In order to produce a high quality, pure sol, frequently organo-metallic precursor compounds of the desired ceramic oxides are mixed and dissolved in a suitable solvent. The resultant solution is then hydrolyzed to form a nano-cluster sol, and subsequently a gel composed of organo-metallic polymers or macro clusters. Additives can be added to control the viscosity and surface tension of the sol-gel. Films are prepared by either spin, dip or spray coating, or painting onto an appropriate substrate. The coated substrate is then fired to remove the water and organic material and to develop the final ceramic structure. The sol-gel process has several advantages over other fabrication methods. It is simple, more economically feasible and permits coating of complex geometries, not necessarily limited to line of sight.
U.S. Pat. No. 4,614,673 discloses the use of aluminum isopropoxide or aluminum secondary butoxide for the sol-gel ceramic deposition on a substrate. This patent discloses that a gelled film can be deposited directly on a sensitive substrate to obtain protective ceramic coating on the substrate. An alumina sol, for example, can be sprayed through a flowing stream of ammonia to create a uniformly thick film, which adheres to metals, plastics, or, in some cases, water soluble materials. The film can then be cured, usually at relatively low firing temperatures, to complete the coating. One specific method for forming a protective alumina coating on the surface of the sensitive substrate, such as on a metal, a metal matrix composite or a plastic which is not wet by a conventional alumina sol, comprises the steps of (a) spraying an alumina sol containing alumina in isopropanol on the surface through a flowing stream of anhydrous ammonia to deposit a gelled film directly on the surface; (b) continuing the spraying to build up a substantially uniform film of the desired thickness; and (c) drying and curing the film at a temperature between 200 and 600.degree. C. to complete the protective alumina coating.
In the field of ceramics, sol-gel processing offers many advantages over conventional ceramic processing, including high purity, homogeneity and low sintering temperatures of the ceramics. However, ceramics produced by this sol-gel process have a number of limitations which restrict the use of such ceramics for corrosion and wear protection. Such problems include cracking, weak bonding between coating and substrate, and permeability to gases and liquids.
Some of these problems have recently been partially addressed in an application of modified sol-gel technology for deposition of thick ceramic coatings by Barrow et al., U.S. Pat. No. 5,585,136, granted Dec. 17, 1996, which discloses a method for producing thick ceramic films of greater than 10 microns on selected substrates. Conventional sol-gel solutions are loaded with up to 90% by weight of finely divided ceramic particles and mixed. The resulting slurry or paint can be either spun or dip-coated or sprayed or painted onto a planar or other substrate, fired to remove the organic materials and to develop a microcrystalline structure. The fired film may then be heated. Composite films are also contemplated. The thicker coatings do not crack upon drying because the gel phase contains up to about 90% weight of the filler calcine ceramic. However, although the coatings were heat treated at temperatures up to 1000.degree. C., residual porosity persisted, giving relatively low hardness and high permeability to liquids.
In order to exploit the desired properties of the ceramic, it is essential that the ceramic film be dense, not permeable to gases and liquids, and crack-free. Sol-gel films are, however, very susceptible to substrate interaction, and defects and stresses within the film coating. As a general rule, the thinner the film, the lower the internal stresses and the number of defects.